Have reviewed your install further and found the cause to be the Joomla system plugin "System - Joomailer Mailchimp Signup" and have disabled this plugin within Extensions > Plugin Manager. Once disabled was able to confirm saving users and changing usertype at same time is now functional. Please also confirm.

Joomla system plugins execute on every page and CB plugins load on every CB related page. So it had to be either of the two. You basically disable them 1 by 1 testing after each disable and see if resolved and if isn't resolved simply re-enable what you disabled and continue onwards. It's a time consuming process but usually always works to find the conflict.